1. Field of the Invention
This invention relates to optical devices, such as Light Emitting Diodes (LEDs) and Laser Diodes (LDs), grown on templates that modulate strain in active layers, thereby modulating the active layer's band structure and polarization of emitted light.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
In references [1-3], it was shown that the presence of strain in quantum wells (QWs) can modulate the band structure of QWs (polarization of spontaneous emission and gain). This is a well-known phenomenon (see, for example, [4]). Generally, strain in semipolar nitride epitaxial layers, with hexagonal wurtzite crystal structure, is anisotropic due to the different lattice parameters, a and c (lattice anisotropy). Reference [5] reports the following values for lattice constants: a(AlN)=3.112 Angstroms, a(GaN)=3.189 Angstroms, a(InN)=3.54 Angstroms, c(AlN)=4.982 Angstroms, c(GaN)=5.185 Angstroms, and c(InN)=5.705 Angstroms.
However, this strain-anisotropy is automatically determined by the difference of lattice constant between a considered epitaxial layer and the substrate on which the considered layer is coherently grown. Therefore, prior to the present invention, there was no way to control anisotropy of strain in QWs.
FIG. 1 illustrates the co-ordinate system used in Yamaguchi's study [1], where X2 is the c-axis projection and θ indicates the orientation of the substrate (e.g., θ=0 corresponds to a c-plane substrate). FIGS. 2(a)-(c) illustrate substrate orientation dependences of the in-plane luminescence polarization degree for unstrained QWs, thick strained GaN films, and compressively strained QWs with isotropic in-plane biaxial strain. The quantum confinement effect due to thin QWs causes luminescence polarization parallel to X2, as shown in FIG. 2(a). Compressive strain, on the other hand, causes luminescence polarization parallel to X1, as shown in FIG. 2(b). FIG. 3 illustrates substrate orientation dependences of the in-plane luminescence polarization degree, for coherently grown In0.3GaN QWs on GaN substrates. Thus, FIGS. 2 and 3 illustrate different luminescence polarizations or band structures that result from different strain (e.g., anisotropic strain) situations. In these calculations shown in FIGS. 2 and 3, the differences in lattice constants are assumed as 3.3% and 3.0% for the a and c lattice parameters, respectively.
Thus, if strain anisotropy can be modulated, as shown in the present invention, optical properties in LEDs/LDs can be changed with a high degree of freedom.